Brown spot disease caused by Alternaria alternata (Fr.: Fr.) Keissl is a serious problem forproduction of tangerines and tangerine hybrids in Iran. The Tangerine pathotype causes brownspot disease on young leaves and immature fruits of limited varieties of mandarins and tangerines(Citrus. reticulata Blanco). Specificity in the interaction between tangerine and the pathogen isdetermined by a host- specific toxin (HST), called ACT-toxin. Culture filtrates of isolates ofAlternaria alternata were selectively toxic to tangerine leaves; related toxic compounds wereisolated from culture filtrates of Alternaria alternata. The biological characteristics of ACT-toxinmatched the criteria for HST. ACT-toxin was purified by chromatography analysis, and H-NMRdata showed the presence of ACT-toxin related structures. These results suggest that collectedisolates produce ACT-toxin which is toxic to specific host plants (tangerines). This host specifictoxin rapidly affects plasma membrane integrity of susceptible genotypes and plays a critical rolein the infection process, pathogenicity and host specificity of the pathogen.